Someone at my work recently accidentally opened the office door using their Oyster card - yes the last 8 digits that the door reader uses actually matched a valid office pass!
Used to work at an access control manufacturer that supported different reader technologies, so I had various test cards. For mag swipe cards, I would use a Tesco loyalty card for example.
Once at a high profile end user site in London that used bar codes to produce temporary visitor cards. For my bar code tester, I had a piece of card that had a barcode and label from a "Tesco Chicken Sandwich" package. Our customer who I was working with was not pleased to see me using part of my lunch to gain access and bloody nicked it off me. Bastard